Blisters are small pockets of fluid that usually form in the upper layers of skin after it’s been damaged. The fluid inside a blister can be clear (serum), blood, or pus, depending on the cause of the blister and whether it has become inflamed or infected. Most blisters are filled with clear fluid (serum) . Blood blisters are filled with blood, and pus-filled blisters occur when the blister becomes infected. The fluid inside a blister provides natural protection for the skin beneath it, and most blisters heal naturally after three to seven days without medical attention. It’s important not to burst a blister, as this could lead to an infection or slow down the healing process. If a blister gets infected, it will fill with milky-white pus.